In a high intensity discharge (HID) lamp, a medium to high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas. One class of HID lamps comprises electrodeless lamps which generate an arc discharge by generating a solenoidal electric field in a high-pressure gaseous lamp fill. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, resulting in a toroidal arc discharge in the arc tube.
In developing high-efficiency RF circuits to drive an electrodeless lamp, such as an electrodeless HID lamp, it is desirable to accurately determine the values of the plasma impedance and the coupling coefficient between the excitation coil and the lamp. Of course, since there are no electrodes, the impedance cannot be directly determined using arc voltage and current measurements. Therefore, it is desirable to provide an indirect method for measuring the plasma impedance and furthermore to provide a simulated load circuit for designing and testing ballast circuits for electrodeless discharge lamps.